A field expander kit is available for 3" diameter Saf-T Vent installations. The field expander is a tool the installer can use to recreate the male taper on a cutoff piece of pipe.
Heat Fab 73EXPKIT - Field Expander Kit
A field expander kit is only...

Into-the-Flue-Collar Appliance Adapter - A generic flue collar adapter used on appliances with vent outlets with a nominal inside diameter. The crimped portion of this adapter is inserted into the flue collar while the hose clamp is tightened around the...